(1) Field of the Invention
The present invention relates to a digital receiving system for electromagnetic signals.
(2) Description of the Prior Art
In the field of signal acquisition systems, there is a need for jam-resistant wideband multifunction digital systems.
To address this need, U.S. Pat. Nos. 6,466,167 and 7,250,920 disclose segmenting an air interface as part of a communications system.
In Steinbrecher (U.S. Pat. No. 6,466,167), an antenna apparatus is disclosed in which the apparatus comprises an array of antenna elements with each element having a phase center. An observable signal is generated in which the signal contains a low frequency component and a high frequency component is generated. The high frequency component is summed with the signal received by each antenna element being near a phase center.
As described in the reference, these signals are fed into a signal processing arrangement that processes the signals with the low frequency component of the observable signal, including analog-to-digital conversion, in order to: (i) remove the high frequency component of the observable signal; (ii) normalize the effects of the signal transfer characteristics on the digital sum signals; (iii) synchronously re-sample the digital sum signals; and (iv) differentially time reference each digital sum signal to the phase center of the corresponding antenna element. The processed digital signals are combined into a single composite signal.
The Steinbrecher patent also teaches a method for conditioning and recombining a plurality of digital channels without the loss of dynamic range or system sensitivity. The linear dynamic range of digital signal processors designed to combine multiple digital data streams is related to the way that numerical values are represented in the digital signal processor. Floating point processors represent numerical values by two digital values that represent integers: 1) a power of ten known as the ‘characteristic’ or ‘exponent’ and 2) a number between 0 and 10 that is known as the ‘mantissa’ or ‘significand’. Digital signal processor linear dynamic range is related to the number of bits reserved for representing the mantissas.
Generally, the signal dynamic range of the digital signal processor is at least 5 dB for each mantissa bit, which implies that a 24 bit mantissa representation would lead to an approximately 120 dB signal dynamic range. Since the number of bits assigned to each mantissa is a programmable value, a digital signal processor can be programmed to have an adequate signal dynamic range to linearly combine a plurality of analog-to-digital converter (ADC) data streams. Each time that the number of ADCs is doubled, the digital signal processor dynamic range requirement increases by 6 dB or one bit. Thus, it is prudent to add at least two bits to each mantissa representation for each time that the number of ADCs is doubled.
The Steinbrecher reference further teaches that when the digital data streams produced by (2)exp(Y) ADCs are linearly combined, the Third Order Intercept (TOI) of the system increases by (3×Y)dB. Since the noise figure of the system is nearly unaffected by the multiple conversion process, the spurious-free dynamic range (which is proportional to the ⅔ power of the TOI) increases by (2×Y)dB. The resulting increase in TOI is demonstrated in the example of two narrowly spaced equal-amplitude frequency tones that are commonly called a ‘two-tone’.
Assume that a two-tone signal is applied through a M-way power divider to a parallel array of M ADCs such that the two-tone peak voltage is approximately equal to the full scale ADC voltage in each ADC and further assume that the resulting input third-order intercept is TOI(M). Next, assume that two such M-arrays are combined with a two-way power divider.
In order to achieve the same signal level in each of the 2M ADCs, the input two-tone power level must be increased by 3 dB. Assuming that the third-order inter-modulation terms remain coherent, the input-referred inter-modulation ratio will remain the same in the 2M-array—as was observed in the M-array. It follows that the TOI(2M), which is the input level expressed in dB plus one-half of the inter-modulation ratio expressed in dB, will be 3 dB greater than TOI(M)[expressed as an equation TOI(2M)=2×TOI(M)].
Since the noise is uncorrelated across the 2M ADC channels, the noise figure will remain the same for the 2M array as for the M-array—except for a small increase that will result from the loss of the two-way power divider. It also follows that the spurious-free dynamic range, which is proportional to the ⅔ power of the TOI, will increase by 2 dB.
In Steinbrecher (U.S. Pat. No. 7,250,920), an air interface metasurface is described that efficiently captures incident broadband electromagnetic energy and provides a method for segmenting the total metasurface capture area into a plurality of smaller capture areas such that the sum of the capture areas is equal to the total capture area of the metasurface. The segmentation of the electromagnetic capture area is analogous to the segmentation of the focal plane of a digital camera into pixels. Thus, the equal area segments of the air interface are called “epixels”.
The segmentation property of the Steinbrecher metasurface is used to improve the performance of a digital receiving system that is designed to capture small electromagnetic signals in a wideband electromagnetic environment which is dominated by large electromagnetic interfering signals. It is demonstrated that the fundamental performance limits of a wideband high dynamic-range digital receiving system are directly determined by the ADC converter properties. Also, it can be shown that the dynamic range of a digital receiving system can be significantly greater than the dynamic range of analog-to-digital converters and can be increased to meet mission dynamic-range requirements by increasing the numbers of analog-to-digital converters in the system and by using the segmentation property of the Steinbrecher metasurface.
The electromagnetic radiation interface is suitable for use with radio-wave frequencies. A surface is provided with a plurality of electrically-conductive segmentation elements. A corresponding plurality of termination sections are provided so that each element pair is terminated with a termination section. The termination section may comprise one or more different termination packages for operating on received electromagnetic radiation and/or producing desired reflections and transmissions. In one embodiment, switches and termination packages may be implemented within integrated circuits wherein the switches may be utilized to switch between different termination packages. The termination packages may include selected fixed reactances and/or modulators designed to produce desired reflections (e.g. Doppler effects give the impression that the surface of the interface is moving at a speed different than the actual speed).
The range of signal amplitudes in a broadband electromagnetic environment can be significantly greater than the dynamic range of commercially-available ADCs. ADCs are limited to be used as RF-to-Digital converters in wideband software-defined radio applications. One way to resolve this problem is to limit the analog bandwidth of the software-defined radio pre-amplifier or to automatically limit the gain of the analog pre-amplifier that precedes the ADC. Another way to resolve this problem is to incorporate an analog beamforming apparatus in order to reject the large signals before the signals can interfere with the analog-to-digital conversion process—which implies dynamic control of the analog beamforming system. In either case, the result is a comparatively costly system incorporating limited-use components.
Commercial ADCs are used in numerous applications. ADC cost is steadily decreasing as the costs follow industry standard learning curves while ADC performance is steadily increasing as silicon processes continue to advance in innovation. However, the dynamic-range performance of commercial ADCs increases at a slower rate because the bulk of commercial applications can be met with current technology.
The Steinbrecher reference teaches a method for reducing the signal level applied to each ADC while maintaining a wide bandwidth for receiving signals and while maintaining adequate sensitivity to receive the smallest desired signals in the presence of large in-band interfering signals. The segmented metasurface of the Steinbrecher reference provides an efficient method for managing the signal level in a plurality of signal channels.
The partitioned aperture provides a capability to limit the power captured and fed to each ADC. Each ADC receives the power captured by one epixel while the system receives the power captured by the eplane comprising the sum of all epixels. Thus, the capture area of one epixel establishes the maximum incident power level for the system while the total eplane capture area establishes the minimum detectable signal level for the system. Since the epixel and eplane capture areas are independent of frequency, the instantaneous bandwidth can be as large as the Nyquist bandwidth of the ADCs—which is the primary bandwidth objective.
In addition to the improvements of the cited references, a preference still exists for a procedure to optimize the design of the partitioned air-interface metasurface. The optimized procedure could be based on the performance of a specific ADC and on mission environment signal parameters. ADC performance parameters and the mission environmental parameters comprising the largest signal to be tolerated and the smallest signal to be received will bound and determine the optimum system design parameters.